1. Technical Field
The present invention relates to IEEE 802.3 networks having a buffered distributor as a full-duplex repeater, more specifically to arrangements for outputting flow control frames from the buffered distributor to corresponding network nodes.
2. Background Art
Efforts are currently underway to define standards for the operation of shared and full-duplex gigabit networks according to IEEE 802.3 protocol. Two modifications are currently in progress by the IEEE 802.3z task force to make Gigabit Ethernet viable, namely (1) carrier extension in which the slot time is increased to 512 bytes (4096 bits) without increasing the minimum frame length, and (2) frame bursting in which a station sends several frames separated by the extend carrier signals in a single burst. A network node (i.e., a network station) performs frame bursting by attempting to send a first packet according to the conventional Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol. The network node starts a burst timer at the beginning of the transmission of the first packet. If the first packet transmission is successful and if the network node has a second data packet (i.e., frame) to send, the network node transmits a second data packet if the burst timer has not expired. The network station also maintains the carrier sense in an asserted state (e.g., holds carrier sense high) during the entire burst. Hence, the network station can transmit a burst of data packets until the burst timer expires or until the network station has no additional data packets to transmit. If a collision occurs during transmission of the first frame, the station follows the conventional CSMA/CD protocol, namely jam, abort, and collision mediation using the truncated binary expediential backoff (TBEB) algorithm.
Certain vendors have proposed a buffered distributor, also referred to as a full-duplex repeater, as a viable alternative to a full-duplex switch. Specifically, the buffered distributor includes a number of network ports, where each network port is configured for full-duplex communication with a corresponding network node. Each network port includes a receive first in-first out (FIFO) buffer, and a transmit FIFO buffer. The buffered distributor also includes a repeater core, also referred to as a "backplane", that distributes a data packet stored in one of the receive FIFO buffers to the other network ports. Specifically, a receive FIFO buffer outputs a stored data packet to the other network ports by gaining access to the backplane. Access to the backplane maybe implemented either by CSMA/CD logic, where the Receive FIFO buffers contend for access to the backplane using a zero topology collision domain (using CSMA/CD logic), or another arbitration scheme such as round robin.
The above-described buffered distributor also includes a flow control mechanism according to IEEE 802.3x, where the repeater core will generate a pause frame when a predefined threshold is reached in the one of the receive FIFO buffers. Specifically, a network port having a detected congestion condition (i.e., stored data exceeding the predefined threshold) in its corresponding receive FIFO buffer will output a MAC control frame carrying a predetermined Pause interval into the corresponding transmit FIFO buffer. The network node in communication with that network port, having received the MAC control frame, will suspend transmission for the predetermined Pause interval, enabling the repeater core to remove packet data from the congested receive FIFO buffer.
A problem encountered in designing the buffered distributor involves the sizing of the receive FIFO buffers. Specifically, the sizing of the receive FIFO buffers is based upon the ability of the repeater core to efficiently generate flow control messages to the end network stations. Specifically, the use of a large receive FIFO buffer eliminates the necessity of precise flow control between the buffered distributor and the end network stations, while increasing the overall cost of the buffered distributor. Conversely, maintaining relatively small receive FIFO buffer sizes reduces the cost of the buffered distributor, while requiring precise flow control between the buffered distributor and the end network stations. If precise flow control is not maintained in a buffered distributor having relatively small receive buffer sizes, then congestion in the buffered distributor may cause the loss of received data packets. In addition, transmission of flow control frames having excessively tong pause intervals will adversely affect the network throughput by unnecessarily creating idle time after the receive FIFO buffer of the buffered distributor has been cleared.